1. Field of the Invention
The present invention relates to hydraulic systems that independently control the operation of a plurality of hydraulic actuators, and more particularly to such hydraulic systems in which various hydraulic actuators have different operating pressure limits as determined by separate pressure relief valves.
2. Description of the Related Art
Numerous types of machines have components that are moved by a hydraulic system. For example, a lift truck is vehicle for transporting objects throughout a factory or warehouse and has an engine which drives a pump to provide pressurized fluid for powering different functions, such as driving wheels to propel the vehicle or lifting the objects.
An exemplary lift truck 10 is shown in FIG. 1 and includes a body 12 with an operator compartment 14. A multiple section, telescopic mast 16 is attached to the front of the body and includes a base section 18 and one or more telescopic sections 20 nested within the base section. A fork carriage 22 with load carrying forks 23 is slidably mounted to one of the telescopic sections and is moved up and down by a lift cylinder 24. Typically the lift cylinder 24 is connected to a mechanism (not shown) comprising chains which pass over pulleys to extend and retract the telescopic sections 20 relative to the base section 18. A tilt cylinder 26, horizontally mounted between the front wheels 25 of the lift truck 10, is attached to the body 12 and the lower end of the mast base section 18. The tilt cylinder 26 pivots the telescopic mast 16 about a horizontal shaft 28 to tilt the ends of the forks 23 up and down to hold the load thereon. The hydraulic fluid that drives the lift and tilt cylinders 24 and 26 is controlled by valves that are operated by controls in the operator compartment 14.
Use of the lift truck 10 often requires that the lift and tilt cylinders 24 and 26 operate in unison to tilt the mast 16 as the fork carriage 22 is being raised. However, each of those functions has a unique pressure characteristic which dictates that its maximum pressure be limited to a different magnitude. Because the fork carriage 22 can carry a relatively heavy load, the maximum pressure limit for the lift cylinder 24 may be 200 bar, whereas the maximum pressure limit for the tilt cylinder is on the order of 140 bar, for example. These maximum pressure levels are determined by the setting of pressure relief valves at various locations in the hydraulic circuit.
Quite often each hydraulic function on a machine had a separate pressure relief valve that was set to a specific pressure limit for the associated function. This type of control was relatively expensive as a pressure relief valve was required for each function even though several of them had the same pressure limit.
U.S. Pat. No. 4,561,463 describes an alternative hydraulic system that has a multiple section valve assembly with a pair of relief valves, one for a single high pressure function and another for two lower pressure functions. A high pressure relief valve governed the pressure at the inlet to the valve assembly and that of the first valve section for the high pressure function. The pressure in the subsequent valve sections was governed by a second relief valve having a lower setting. However, when the second valve section provided pressurized fluid to its associated hydraulic actuator, the third valve section was rendered inoperative. Thus the second and third valve sections were serially connected and all the hydraulic functions could not operate simultaneously.
Therefore, a need still exists to enable three or more hydraulic functions, that require different pressure limits, to operate simultaneously without having to provide a separate pressure relief valve for each function.